CN111880251B - SPP coupler based on asymmetric metal grating structure and manufacturing method - Google Patents

Abstract

The application discloses an SPP coupler based on an asymmetric metal grating structure and a manufacturing method thereof, wherein the SPP coupler enters a medium grating layer after incident light passes through a medium filling layer, and the asymmetric metal grating structure is arranged to be an inverted L-shaped structure or a Z-shaped structure, so that the incident light can vertically enter a substrate to excite SPP, oblique incidence excitation is not needed, and the complexity of an external light path can be greatly reduced; the medium grating layer is provided with a plurality of periodic medium gratings, the periodic medium grating structure can realize higher optical coupling efficiency, and can better limit incident light in the substrate for one-way transmission, so that the optical coupling efficiency and the one-way ratio can be improved.

Description

SPP coupler based on asymmetric metal grating structure and fabrication method

technical field

The present application relates to the technical field of couplers, and in particular, to an SPP coupler based on an asymmetric metal grating structure and a corresponding fabrication method.

Background technique

Surface Plasmon Polariton (SPP) is a collective oscillation phenomenon of free electrons or bound electrons on the surface of metal structures induced by an external electromagnetic field. interaction of matter. Surface plasmon can localize the incident light in the sub-wavelength region of the metal surface. Compared with the traditional dielectric optical waveguide device, it can break through the limitation of the diffraction limit, thereby further reducing the size of the device. It is of great significance to realize the ultra-small feature size of nanoelectronic devices and ultra-high transmission speed of dielectric optics.

The SPP unidirectional coupler is one of the important components of the integrated optical circuit, which has been paid much attention by researchers. However, how to design metal micro-nano structures so that light can be efficiently coupled to metal micro-nano structures to form unidirectional transmission SPPs is still one of the important problems. In addition, the existing SPP coupler structure is usually complicated in preparation process and requires high equipment conditions, which becomes a major factor restricting its large-scale application. Therefore, reducing the difficulty of production is also another important issue to consider.

SUMMARY OF THE INVENTION

The present application provides an SPP coupler based on an asymmetric metal grating structure and a manufacturing method, which are used to solve the technology that the optical coupling efficiency of the existing SPP unidirectional coupler is lower than that of the SPP unidirectional and the corresponding coupler is difficult to manufacture. question.

In view of this, a first aspect of the present application provides an SPP coupler based on an asymmetric metal grating structure, which is provided with a substrate, a dielectric grating layer, an asymmetric metal grating structure and a dielectric filling layer in sequence from bottom to top;

The dielectric grating layer includes several dielectric gratings arranged at equal distances;

The asymmetric metal grating structure partially covers the surface of the dielectric grating, and the asymmetric metal grating structure is an inverted "L" shape structure or a "Z" shape structure.

Preferably, the refractive index of the dielectric grating is 1.4-1.7, the width of the dielectric grating is 75-200 nm, the height of the dielectric grating is 75-300 nm, and the grating period of the dielectric grating layer is 300-800 nm.

Preferably, the grating period of the dielectric grating layer is 400-600 nm.

Preferably, the thickness of the dielectric filling layer above the surface of the asymmetric metal grating structure is 50-800 nm.

Preferably, the metal degree of the asymmetric metal grating structure is greater than 50 nm, and the metal thickness of the asymmetric metal grating structure is 70-150 nm.

On the other hand, an embodiment of the present application provides a method for fabricating an SPP coupler based on an asymmetric metal grating structure, including the following steps:

S101: forming a dielectric grating layer having a plurality of dielectric gratings arranged at equal distances on a substrate;

S102: Form an asymmetric metal grating structure on the dielectric grating layer, the asymmetric metal grating structure partially covers the surface of the dielectric grating, and at the same time, the asymmetric metal grating structure is an inverted "L" shape structure or "" Z"-shaped structure;

S103: Filling the space where the asymmetric metal grating structure is located with a medium and forming a medium filling layer, the medium filling layer covers the asymmetric metal grating structure and the surface of the dielectric grating layer;

S104: Etch the independent unit of the coupler through an etching process on the sample obtained in the step S103.

Preferably, the manufacturing method for forming the dielectric grating layer in the step S101 adopts any one of the following three methods:

1) First, a photoresist is formed on the upper surface of the substrate by a spin coating method, and then an exposure and development process is used to form a dielectric grating layer having the dielectric grating made of the photoresist;

2) First, a dielectric layer is formed on the upper surface of the substrate, and the dielectric layer is made of SiO2, SiNx, ITO or AlN material, and then a photoresist is formed on the dielectric layer by a spin coating method, and The dielectric grating made of the photoresist is formed by exposing and developing, and finally, the dielectric grating layer is formed by a dry etching process;

3) First, a dielectric layer is formed on the upper surface of the substrate, and the dielectric layer is made of SiO2, SiNx, ITO or AlN material, and then a photoresist is formed on the dielectric layer by a spin coating method, and The dielectric grating made of the photoresist is formed by a nano-imprint process, and finally, the dielectric grating layer is formed by a dry etching process.

Preferably, the asymmetric metal grating structure is formed by an oblique evaporation method in the step S102, which specifically includes: emitting an electron beam through an electron beam crucible, and aligning the sample obtained in the step S101 with a preset tilt angle relative to each other. The outgoing direction of the electron beam is set obliquely, and the preset inclination angle is adjustable. After evaporation, an asymmetric metal grating structure with an inverted "L" or "Z" shape is formed on the surface of the dielectric grating. .

Preferably, the preset inclination angle is 30°˜60°.

Preferably, the specific step of forming the dielectric filling layer in the step S103 includes: forming the dielectric filling layer with a flat surface on the surface of the asymmetric metal grating structure and the dielectric grating layer by a spin coating method, wherein , the rotational speed of the spin coating is 4000-10000 rpm, the number of times of the spin coating is 3-5 times, and the medium of the medium filling layer is hydrogen silsesquioxane.

As can be seen from the above technical solutions, the embodiments of the present application have the following advantages:

An SPP coupler based on an asymmetric metal grating structure provided by an embodiment of the present application, when incident light enters the dielectric grating layer after passing through the dielectric filling layer, the asymmetric metal grating structure is set as an inverted "L" shape structure or "Z" shape ” shape structure, so that the incident light can enter the substrate vertically to excite the SPP without oblique incident excitation, which can greatly reduce the complexity of the external optical path; while the dielectric grating layer has several periodic dielectric gratings, the periodic The dielectric grating structure can achieve high optical coupling efficiency, and can better confine the incident light in the substrate for unidirectional propagation, so that the ratio of optical coupling efficiency to unidirectional can be improved. In addition, the SPP coupler of this embodiment can The structure is simple, the manufacturing difficulty is low, and the utility model is suitable for mass production.

Another embodiment of the present application provides a method for fabricating an SPP coupler based on an asymmetric metal grating structure, which is simple and feasible, reduces fabrication difficulty, and is suitable for mass production. The beneficial effects of the SPP coupler based on the asymmetric metal grating structure of the example are the same.

Description of drawings

1 is a schematic structural diagram of a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by an embodiment of the present application;

2 is a cross-sectional view of a sample in step S301 of Example 1 of a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by an embodiment of the present application;

3 is a cross-sectional view of a sample in step S302 in Example 1 of a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by an embodiment of the present application;

4 is a schematic diagram of evaporation in step S303 in Example 1 of a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by an embodiment of the present application;

5 is a cross-sectional view of a sample in step S303 of Example 1 of a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by an embodiment of the present application;

6 is a top view of a sample in step S303 of Example 1 of a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by an embodiment of the present application;

7 is an SPP energy flow distribution diagram of a sample of Example 1 of a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided in an embodiment of the present application;

FIG. 8 is a cross-sectional view of a sample of Example 2 of a method for fabricating an SPP coupler based on an asymmetric metal grating structure according to an embodiment of the present application.

Detailed ways

In order to make those skilled in the art better understand the solutions of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only It is a part of the embodiments of the present application, but not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

This embodiment provides an SPP coupler based on an asymmetric metal grating structure, which is provided with a substrate, a dielectric grating layer, an asymmetric metal grating structure, and a dielectric filling layer in sequence from bottom to top;

The dielectric grating layer includes several dielectric gratings arranged at equal distances;

The asymmetric metal grating structure partially covers the surface of the dielectric grating, and the asymmetric metal grating structure is an inverted "L" shape structure or a "Z" shape structure.

In this embodiment, when the incident light enters the dielectric grating layer after passing through the dielectric filling layer, the asymmetric metal grating structure is set to be an inverted "L"-shaped structure or a "Z"-shaped structure, so that the incident light can enter vertically. The substrate can excite SPP without oblique incident excitation, which can greatly reduce the complexity of the external optical path; while the dielectric grating layer has several periodic dielectric gratings, the periodic dielectric grating structure can achieve high optical coupling efficiency, and The incident light can be better confined in the substrate for unidirectional propagation, thereby improving the optical coupling efficiency and the unidirectional ratio. In addition, the SPP coupler of this embodiment has a simple structure and low manufacturing difficulty, and is suitable for mass production.

The above is an embodiment of an SPP coupler based on an asymmetric metal grating structure provided by the present application, and the following is another embodiment of an SPP coupler based on an asymmetric metal grating structure provided by the present application.

This embodiment provides an SPP coupler based on an asymmetric metal grating structure, which is provided with a substrate, a dielectric grating layer, an asymmetric metal grating structure, and a dielectric filling layer in sequence from bottom to top;

The dielectric grating layer includes several dielectric gratings arranged at equal distances;

The asymmetric metal grating structure partially covers the surface of the dielectric grating, and the asymmetric metal grating structure is an inverted "L" shape structure or a "Z" shape structure.

Further, the refractive index of the dielectric grating is 1.4-1.7, the width of the dielectric grating is 75-200 nm, the height of the dielectric grating is 75-300 nm, and the grating period of the dielectric grating layer is 300-800 nm.

Further, the grating period of the dielectric grating layer is 400-600 nm.

Further, the dielectric filling layer can play the role of waveguide and improve the coupling efficiency. By limiting the thickness of the dielectric filling layer, the one-way coupling efficiency and one-way ratio of the incident light are limited, and the propagation direction of the SPP is regulated. In this embodiment, The thickness of the dielectric filling layer above the surface of the asymmetric metal grating structure is 50-800 nm.

Further, the substrate may be quartz, mica, PDMS, sapphire or other materials with high transmittance in the wavelength range of 300-1600 nm.

Further, the dielectric material of the dielectric grating can be photoresist, SiO2, SiNx, ITO or AlN.

Further, the metal degree of the asymmetric metal grating structure is greater than 50 nm, and the metal thickness of the asymmetric metal grating structure is 70-150 nm.

The above is another embodiment of the SPP coupler based on the asymmetric metal grating structure provided by the present application, and the following is an embodiment of the manufacturing method of the SPP coupler based on the asymmetric metal grating structure provided by the present application.

For easy understanding, please refer to FIG. 1 , a method for fabricating an SPP coupler based on an asymmetric metal grating structure provided in this embodiment includes the following steps:

S101: forming a dielectric grating layer having a plurality of dielectric gratings arranged at equal distances on a substrate;

It should be noted that the substrate can be quartz, mica, PDMS, sapphire or other materials with high transmittance in the wavelength range of 300-1600nm; meanwhile, the dielectric grating material can be photoresist, SiO2, SiNx, ITO or AlN et al.

S102: forming an asymmetric metal grating structure on the dielectric grating layer, the asymmetric metal grating structure partially covers the surface of the dielectric grating, and at the same time, the asymmetric metal grating structure is an inverted "L" shape structure or a "Z" shape structure;

S103: filling a medium in the space where the asymmetric metal grating structure is located and forming a medium filling layer, and the medium filling layer covers the surface of the asymmetric metal grating structure and the dielectric grating layer;

S104: Etch the independent unit of the coupler through an etching process on the sample obtained in step S103.

It should be noted that step S104 may also be completed after step S101 or step S102.

The manufacturing method provided in this embodiment is simple and easy to implement, reduces the manufacturing difficulty, and is suitable for mass production.

The above is an embodiment of the method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by the present application, and the following is another implementation of the method for fabricating an SPP coupler based on an asymmetric metal grating structure provided by the present application. example.

A method for fabricating an SPP coupler based on an asymmetric metal grating structure provided in this embodiment includes the following steps:

S201: forming a dielectric grating layer having a plurality of dielectric gratings arranged at equal distances on a substrate;

It should be noted that the substrate can be quartz, mica, PDMS, sapphire or other materials with high transmittance in the wavelength range of 300-1600nm; meanwhile, the dielectric grating material can be photoresist, SiO2, SiNx, ITO or AlN et al.

S202: an asymmetric metal grating structure is formed on the dielectric grating layer, the asymmetric metal grating structure partially covers the surface of the dielectric grating, and at the same time, the asymmetric metal grating structure is an inverted "L" shape structure or a "Z" shape structure;

S203 : filling a medium in the space where the asymmetric metal grating structure is located and forming a medium filling layer, and the medium filling layer covers the surface of the asymmetric metal grating structure and the dielectric grating layer;

S204: Etch the independent unit of the coupler through an etching process on the sample obtained in step S203.

Further, the manufacturing method of forming the dielectric grating layer in step S201 adopts any one of the following three methods:

1) First, a photoresist is formed on the upper surface of the substrate by a spin coating method, and then an exposure and development process is used to form a dielectric grating layer with a dielectric grating made of the photoresist;

2) First, a dielectric layer is formed on the upper surface of the substrate. The dielectric layer is made of SiO2, SiNx, ITO or AlN materials. Then, a photoresist is formed on the dielectric layer by spin coating, and photoresist is formed by exposure and development. A dielectric grating made of resist, and finally, a dielectric grating layer is formed by a dry etching process;

3) First, a dielectric layer is formed on the upper surface of the substrate, and the dielectric layer is made of SiO2, SiNx, ITO or AlN materials, and then a photoresist is formed on the dielectric layer by spin coating, and formed by a nano-imprinting process A dielectric grating made of photoresist, and finally, a dielectric grating layer is formed by a dry etching process.

Further, in step S202, an asymmetrical metal grating structure is formed by an oblique evaporation method, which specifically includes: emitting an electron beam through an electron beam crucible, and inclining the sample obtained in step S201 with a preset angle of inclination relative to the outgoing direction of the electron beam Setting, the preset inclination angle can be adjusted, and after evaporation, an asymmetric metal grating structure with an inverted "L" or "Z" shape structure is formed on the surface of the dielectric grating.

It should be noted that the tilt angle is adjusted according to the period and duty ratio of the grating. Preferably, the preset tilt angle is 30°˜60°.

Further, the specific step of forming the dielectric filling layer in step S203 includes: forming a flat surface dielectric filling layer on the surface of the asymmetric metal grating structure and the dielectric grating layer by a spin coating method, wherein the rotation speed of the spin coating is 4000-10000 rpm /min, the times of spin coating are 3 to 5 times, and the medium of the medium filling layer is hydrogen silsesquioxane.

It should be noted that the spin coating method is used to prepare the dielectric filling layer, so that the process is simple and the process compatibility is high.

The following is a partial implementation example of the method for fabricating the dielectric grating and the asymmetric metal grating structure in the SPP coupler based on the asymmetric metal grating structure in this embodiment.

Example 1

The method for fabricating the dielectric grating and the asymmetric metal grating structure in the SPP coupler based on the asymmetric metal grating structure provided in this example includes the following steps:

S301: Referring to FIG. 2, a photoresist 12 is formed on the surface of the quartz substrate 11 by a spin coating method and dried, wherein the thickness of the photoresist is 200 nm;

S302 : Referring to FIG. 3 , a dielectric grating layer having a dielectric grating 21 made of photoresist is formed through double-beam interference exposure and development. Among them, the grating period is 550nm, and the width of the dielectric grating is 200nm;

S303: Referring to FIG. 4, the electron beam crucible 2 is used to emit electron beams for vapor deposition, and the arrow direction is the electron beam direction, the sample is fixed on the inclined fixing device 31, and the sample and the electron beam transmission direction are set relative to a certain inclined angle, At the same time, the dielectric grating layer is arranged in the direction of the electron beam, and at the same time, the angle of the tilting fixing device 31 is continuously adjustable. During evaporation, the tilting fixing device 51 can be adjusted at any time through the external remote control device according to the thickness change of the asymmetric metal grating structure 41. As shown in FIG. 5 and FIG. 6 , an asymmetric metal grating structure 41 with an inverted “L” structure is formed on the dielectric grating layer, covering a part of the dielectric grating 21 , wherein the thickness of the metal layer of the asymmetric metal grating structure 41 is is 200nm.

Finally, according to the structure designed in the above steps, during operation, an external light source (such as a single-wavelength laser) is vertically incident on the surface of the structure, and the "L"-shaped asymmetric metal grating structure converts the external light into SPP and couples it to the substrate Transmission in the bottom without oblique incidence excitation, greatly reducing the complexity of the external optical path.

In addition, the unidirectional transmission effect of the coupler formed in the above steps is simulated, wherein the resonant wavelength corresponding to the coupler is 810 nm, and the light is vertically incident on the surface of the coupler. Figure 7 shows the energy flow distribution of the SPP when the wavelength of the incident light is 810 nm for the periodic dielectric grating structure (because it is a periodic structure, so Figure 7 shows one of the periodic dielectric grating structures), in which the scale on the right side of the figure represents The energy flux intensity of SPP in the x-direction can be seen from Figure 7 that the SPP is mainly distributed in the range of about 300 nm on the surface of the substrate and propagates along the negative x-direction. Compared with ordinary aperiodic symmetric gratings, after the incident light, its energy flow is different in two directions. That is to say, the periodic dielectric grating structure in this example achieves the effect of unidirectional transmission of SPP, and can be better limited to a range of about 300 nm in the substrate for unidirectional transmission. In addition, in the subsequent SPP signal extraction, it is only necessary to break the transmission condition at a specific position, for example, by etching a groove structure on the surface of the substrate, so that the SPP can be emitted through scattered light.

Example 2

For the convenience of understanding, please refer to FIG. 8. The method for fabricating the dielectric grating and the asymmetric metal grating structure in the SPP coupler based on the asymmetric metal grating structure provided in this example includes the following steps:

S401: magnetron sputtering to form a SiO2 dielectric layer on the quartz substrate 61, and the thickness of the SiO2 dielectric layer is 75 nm;

S402: PMMA is formed on the SiO2 dielectric layer by spin coating, and the thickness of PMMA is 100 nm;

S403 : forming a PMMA dielectric grating by nano-imprinting, and then using PMMA as a mask to form a dielectric grating layer with a dielectric grating 62 made of SiO2 material by ICP etching;

S404 : For example, by adopting an oblique evaporation method, a "Z"-shaped asymmetric metal grating structure 63 is formed on the dielectric grating 62 , wherein the thickness of the metal layer of the asymmetric metal grating structure 63 is 75 nm.

In the several embodiments provided in this application, it should be understood that the disclosed apparatus and method may be implemented in other manners. For example, the apparatus embodiments described above are only illustrative. For example, the division of the units is only a logical function division. In actual implementation, there may be other division methods. For example, multiple units or components may be combined or Can be integrated into another system, or some features can be ignored, or not implemented. On the other hand, the shown or discussed mutual coupling or direct coupling or communication connection may be through some interfaces, indirect coupling or communication connection of devices or units, and may be in electrical, mechanical or other forms.

The units described as separate components may or may not be physically separated, and components displayed as units may or may not be physical units, that is, may be located in one place, or may be distributed to multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution in this embodiment.

In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically alone, or two or more units may be integrated into one unit. The above-mentioned integrated units may be implemented in the form of hardware, or may be implemented in the form of software functional units.

As mentioned above, the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand: The technical solutions described in the embodiments are modified, or some technical features thereof are equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions in the embodiments of the present application.

Claims (8)

1. The SPP coupler based on the asymmetric metal grating structure is characterized in that a substrate, a medium grating layer, the asymmetric metal grating structure and a medium filling layer are sequentially arranged from bottom to top;

the medium grating layer comprises a plurality of medium gratings which are arranged at equal intervals;

the asymmetric metal grating structure partially covers the surface of the medium grating, and is of an inverted L-shaped structure or a Z-shaped structure;

the refractive index of the medium grating is 1.4-1.7, the width of the medium grating is 75-200 nm, the height of the medium grating is 75-300 nm, and the grating period of the medium grating layer is 300-800 nm;

the thickness of the medium filling layer relative to the structure above the surface of the asymmetric metal grating structure is 50-800 nm.

2. The SPP coupler based on the asymmetric metal grating structure as recited in claim 1, wherein the grating period of the medium grating layer is 400-600 nm.

3. The SPP coupler based on the asymmetric metal grating structure of claim 1, wherein the metal degree of the asymmetric metal grating structure is larger than 50nm, and the metal thickness of the asymmetric metal grating structure is 70-150 nm.

4. The manufacturing method of the SPP coupler based on the asymmetric metal grating structure is characterized by comprising the following steps of:

s101: forming a medium grating layer with a plurality of medium gratings arranged at equal intervals on a substrate, wherein the refractive index of the medium grating is 1.4-1.7, the width of the medium grating is 75-200 nm, the height of the medium grating is 75-300 nm, and the grating period of the medium grating layer is 300-800 nm;

s102: forming an asymmetric metal grating structure on the medium grating layer, wherein the asymmetric metal grating structure partially covers the surface of the medium grating, and meanwhile, the asymmetric metal grating structure is an inverted L-shaped structure or a Z-shaped structure;

s103: filling a medium in the space where the asymmetric metal grating structure is located and forming a medium filling layer, wherein the medium filling layer covers the asymmetric metal grating structure and the surface of the medium grating structure, and the thickness of the medium filling layer relative to the structure above the surface of the asymmetric metal grating structure is 50-800 nm;

s104: and etching the independent unit of the coupler by an etching process on the sample obtained in the step S103.

5. The method for manufacturing an SPP coupler based on an asymmetric metal grating structure as claimed in claim 4, wherein the method for manufacturing the medium grating layer in step S101 adopts any one of the following three methods:

1) firstly, forming photoresist on the upper surface of the substrate by a spin coating method, and then forming a medium grating layer with the medium grating made of the photoresist by adopting an exposure and development process;

2) firstly, forming a dielectric layer on the upper surface of the substrate, wherein the dielectric layer is made of SiO2, SiNx, ITO or AlN materials, then forming photoresist on the dielectric layer by a spin coating method, forming the dielectric grating made of the photoresist by exposure and development, and finally forming the dielectric grating layer by a dry etching process;

3) firstly, forming a dielectric layer on the upper surface of the substrate, wherein the dielectric layer is made of SiO2, SiNx, ITO or AlN materials, then forming photoresist on the dielectric layer through a spin coating method, forming the dielectric grating made of the photoresist through a nano-imprinting process, and finally forming the dielectric grating layer through a dry etching process.

6. The method for manufacturing an SPP coupler based on an asymmetric metal grating structure according to claim 4, wherein the step S102 is performed by an oblique evaporation method, and specifically includes: and emitting an electron beam through an electron beam crucible, obliquely arranging the sample obtained in the step S101 relative to the emergent direction of the electron beam at a preset inclination angle, wherein the preset inclination angle is adjustable, and after evaporation, forming an asymmetric metal grating structure with an inverted L-shaped or Z-shaped structure on the surface of the medium grating.

7. The method of claim 6, wherein the predetermined tilt angle is 30 ° to 60 °.

8. The method for manufacturing an SPP coupler based on an asymmetric metal grating structure as claimed in claim 4, wherein the step S103 of forming the dielectric filling layer comprises the following steps: and forming the medium filling layer with a smooth surface on the surface of the asymmetric metal grating structure and the surface of the medium grating layer by a spin coating method, wherein the spin coating speed is 4000-10000 r/min, the spin coating frequency is 3-5 times, and the medium of the medium filling layer is hydrogen silsesquioxane.

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